HEAT-TRANSFER BETWEEN FLUID AND PARTICLES IN ASEPTIC PROCESSING

The particle heat transfer coefficient, h(fp), is one of the critical factors, besides the residence time distribution, that has to be known for the design and development of continuous heat processes of particulate food products. In the present work, data and methods for determining h(fp) for conditions relevant to aseptic processing have been studied. The influence of product characteristics, such as size and composition of particles, fluid properties, temperature differences and fluid motion, have been investigated. Among the parameters investigated, the flow field around the particles was shown to be the most important for heat transfer but for special conditions the particle size, particle shape or thermal properties of the particle, as well as the apparent viscosity and the thermal properties of the fluid, are equally important for the value of h(fp). For safety considerations, the present investigation indicates that h(fp) values corresponding to an Nu value of 5-6 should be relevant for medium-sized spherical particles transported by a highly viscous liquid. For the quality aspects, different conditions have to be considered. Particles at different positions in the tube flow will experience different heat transfer. The few particles in the centre of the tube will experience a low heat transfer and h(fp) values corresponding to Nu values ranging between 5 and 10. The larger amounts of particles are transported in the liquid with a pronounced velocity gradient which yields Nu values of 10-17, while for extreme conditions occurring for sedimented particles, Nu values between 20 and 45 could be expected